Frequent need arises for bringing about relative synchronization between different sources of information. For example, it often happens that a TV program source is, say, one-half field later than another, and/or the time relationship may drift slowly. At present there is no way to correct the difference so that different sources can be used on the same program. Moreover, there are several TV standards in the world. U.S. and Japan use 525 lines, 60 fields per second, the NTSC standard. In Europe, a common system is the 625 lines, 50 fields system of PAL. A simple system does not exist for converting from one frequency standard to another.
Also, systems employed for delayed broadcast or "stop action" can be relatively complicated and costly, as when magnetic recording and playback apparatus is employed. Moreover, with magnetic recording and playback, generally a minimum delay is required between recording and playback so that the respective recording and playback heads can be physically accommodated along the magnetic recording medium.
Optical recording and playback of digital information advantageously solves some of the foregoing problems, but digital recording can involve close mechanical tolerances which make it difficult to take proper advantage thereof.